Tao Te Ching by Lao-Tze — Read

Thirty spokes unite in one nave and on that which is non-existent [on the hole in the nave] depends the wheel's utility. Clay is moulded into a vessel and on that which is non-existent [on its hollowness] depends the vessel's utility. By cutting out doors and windows we build a house and on that which is non-existent [on the empty space within] depends the house's utility.

Therefore, existence renders actual but non-existence renders useful.